Mebtu Bihonegn Beza

An Adaptive Power Oscillation Damping Controller by STATCOM With Energy Storage

This paper deals with the design of an adaptive power oscillation damping (POD) controller for a static synchronous compensator (STATCOM) equipped with energy storage. This is achieved using a signal estimation technique based on a modified recursive least square (RLS) algorithm, which allows a fast, selective, and adaptive estimation of the low-frequency electromechanical oscillations from locally measured signals during power system disturbances. The proposed method is effective in increasing the damping of the system at the frequencies of interest, also in the case of system parameter uncertainties and at various connection points of the compensator. First, the analysis of the impact of active and reactive power injection into the power system will be carried out using a simple two-machine system model. A control strategy that optimizes active and reactive power injection at various connection points of the STATCOM will be derived using the simplified model. Small-signal analysis of the dynamic performance of the proposed control strategy will be carried out. The effectiveness of the proposed control method to provide power oscillation damping irrespective of the connection point of the device and in the presence of system parameter uncertainties will be verified through simulation and experimental results.

A Modified RLS Algorithm for Online Estimation of Low-Frequency Oscillations in Power Systems

A number of methods have been proposed and implemented to improve system damping against low-frequency electromechanical oscillations in the power system. Among these, flexible ac transmission systems (FACTS) can be used to provide power oscillation damping (POD) function to the power system. The design of the POD algorithms in these devices requires estimation of the power oscillation frequency components and this is mostly achieved through the use of various filter combinations. However, these filter-based solutions are characterized by low bandwidth to extract the required signal components accurately, and this limits the dynamic performance of the FACTS controllers. Moreover, the filters are designed for specific frequencies, and a change in the system would reduce the performance of the methods. Thus, there is a need for a better estimation algorithm with fast and selective estimation of the required signal that is robust against system parameter uncertainties. In this paper, this is achieved by the use of a recursive least square algorithm that uses variable forgetting factor and a frequency adaptation mechanism. The investigated method has fast estimation in transient conditions without compromising its selectivity in steady state. The effectiveness of the proposed method is proven through simulation as well as experimental verification.

Three-level converters with selective Harmonic Elimination PWM for HVDC application

In a voltage source converter (VSC) based HVDC system, the modulation scheme used is an important factor in achieving a desired harmonic performance with allowable semiconductor losses. In this paper, the use of selective Harmonic Elimination Pulse Width Modulation (HEPWM) for a three-level Neutral Point Clamped (NPC) converter in VSC based HVDC application will be discussed. Steady state performance of the converter system in terms of harmonics and losses will be evaluated using MATLAB and PSCAD/EMTDC simulation. Simulation results show a 37% improvement in total semiconductor loss with better harmonic performance by using the three-level solution compared to the two-level solution with the stated modulation scheme.

Improved discrete current controller for grid-connected voltage source converters in distorted grids

This paper deals with improvements to the classical Proportional-Integral (PI) current controller for Voltage Source Converter (VSC) connected to a grid in the presence of harmonics. A discrete PI current controller implemented in the synchronous reference-frame has simple design procedure and robust performance in ideal grid conditions. However, low-order harmonics in the connecting grid will result in a steady state harmonic current flow between the converter and the grid. A method based on harmonic estimation to avoid the steady state harmonic injection to the grid will be discussed. The effectiveness of the described control method will be validated through simulation and experimental verification.

Power oscillation damping controller by static synchronous compensator with energy storage

This paper deals with a control strategy for power oscillation damping using Static Synchronous Compensator (STATCOM) with energy storage. First, analysis of power oscillation damping by only active and reactive power injection into the power system will be carried out using a single machine infinite bus and two machines system model. A control strategy that optimizes the use of active and reactive power injection for power oscillation damping based on locally measured signals will be derived. Small signal analysis of the dynamic performance of the proposed control strategy will be made. The effectiveness of the proposed control method to provide power oscillation damping irrespective of the connection point of the device will be verified by simulation.

Application of Recursive Least Square (RLS) algorithm with variable forgetting factor for frequency components estimation in a generic input signal

Signal estimation is important for protection, system study and control purposes. This paper deals with the application of a Recursive Least Square (RLS) algorithm with variable forgetting factor for estimation of frequency components in a generic input signal. Application areas such as synchronization and control in three-phase and single-phase systems for various grid conditions as well as estimation of harmonic and subsynchronous components will be discussed. Simulation and experimental results will be used to evaluate the dynamic performance of the algorithm.

Analytical Derivation of the AC-Side Input Admittance of a Modular Multilevel Converter With Open- and Closed-Loop Control Strategies

This paper deals with the derivation of the ac-side input admittance of a grid-connected modular multilevel converter (MMC) under both open- and closed-loop ac-side current control strategies. The contribution of the internal dynamics of the MMC on the input admittance depends on the type of ac-side controllers implemented. For this reason, the input admittance is first derived with the MMC operating in open loop on the ac side. The results are then modified to include various ac-side controllers in order to analyze the impact of the internal dynamics of the MMC on the total input admittance. Based on the findings, recommendations are given on modeling the input admittance of the MMC. Finally, the derived analytical models are verified using a detailed switching model of the MMC in the frequency domain.

Independent damping control of multimode low-frequency oscillations using shunt-connected FACTS devices in power system

One of the problems encountered in a power system and that impacts its stable operation is low-frequency electromechanical oscillations. Depending on the system configuration, its operating conditions and the nature of the disturbance, one or more oscillation modes can be excited in the system. This paper deals with the design and analysis of an independent damping controller for multiple oscillation modes using shunt-connected FACTS devices. This is achieved using a selective and adaptive estimation of the various oscillation modes from locally measured signals. The effectiveness of the proposed control method to provide independent damping for the various oscillation modes in the presence of system parameter uncertainties will be verified through small-signal analysis and simulation using both single and multiple compensators.

Impedance Analysis of Modular Multi-Level Converters Connected to Weak AC Grids

This paper investigates the stability of a modular multi-level converter (MMC) based HVDC system connected to a weak ac grid. Impedance analysis is used to study the interactions between the weak ac grid and the converter. For this, the impedance of the MMC is first derived considering various outer- and inner-loop controllers. The Nyquist-stability criterion is then used to analyze the stability of the MMC connected to the weak ac grid for various case studies. The impact of circulating current controller on the stability of the MMC connected to the weak ac grid is also validated for two different types of outer-loop controllers. Finally, time-domain simulations are carried out using MATLAB/Simscape Power System toolbox to validate the analytical results.

Comparison of two control approaches for stability enhancement using STATCOM with active power injection capability

The application of two control strategies for shunt-connected FACTS devices to enhance the transient stability of a power system will be discussed in this paper. The main focus will be on the use of active power in the FACTS device and therefore a STATCOM with active power injection capability will be considered. The first approach represents the classical cascade controller that consists of an inner vector-current controller and various outer-control loops. The alternative approach controls the converter in a similar fashion as a synchronous generator, where the mechanical behavior of the machine is emulated in the control structure using the active power injection capability of the device. Using various simulation and experimental tests, the two methods will be compared in terms of their advantages and disadvantages, and a recommendation will be made on the use of the methods for transient stability enhancement.